FIG. 1 provides the viscosity curves (plotted on a Log scale) as a function of temperature for three polycarbonate grades having different molecular weight and molecular weight distribution (polydispersity). The viscosity measured is the complex viscosity ETA*, obtained with a parallel plate rheometer (Rheometrics RDAII), at constant oscillation frequency (10 rad/sec). Grade 1 is a high flow resin for compact disks and thin-wall molding applications; its molecular weight is the lowest of the three grades. Its mechanical performance is also the worst. Grade 2 is a general purpose polycarbonate with a molecular weight average above the critical molecular weight for entanglements, and Grade 3 is a branched polymer with high melt elasticity used in blow molding. One sees, in first approximation, that the viscosity at any given temperature is shifted by an amount scaled by the respective glass transition temperature,Tg, of the grades. By blending these three grades in any specific proportion, controlled by the Tg of the mix, one has the possibility to "custom fit" the viscosity curve for a given application. This clearly illustrates how the resin suppliers have succeeded in providing the plastic industry with means to lower the viscosity to ease up processing or increase melt elasticity by blending manipulation. The melt index is larger for resins which flow better. The molecular weight of the higher melt index resins is reduced, which explains the lower viscosity. The problem with that solution is that the mechanical performance of the lower molecular weight polymers is also severely reduced, a compromise for better processability which processors have to pay.
The industry would welcome a process which allows the decrease of viscosity of plastic melts without the need to change the molecular weight of the resins, with the added advantage of a reduction of the number of grades a resin manufacturer has to offer.
Shear thinning of plastic materials is well known and is used practically to lower the viscosity of melts during the filling stage of injection molding by increasing the speed of the injecting piston. This is particularly useful in the case of thin wall injection molding where considerable forces are required to fill the mold when the viscosity of the melt remains quasi-Newtonian. Rheologists essentially use two types of instruments to characterize the flow behavior of fluids: capillary rheometers and rotational shear viscometers. In the latter, either a true rotational motion or an oscillation is imparted to the melt, leading to the knowledge of either the steady shear viscosity or the complex viscosity, ETA*. It is well known to rheologists that plots of the complex viscosity, ETA*, versus w, the angular frequency, are similar to plots of viscosity versus shear rate, the so-called Cox Merz's rule.
It is also well known that shear thinning can be obtained, at a given temperature, by either increasing the shear rate or the frequency of oscillation of the melt at constant amplitude of oscillation. For example, the viscosity of PMMA at 239.degree. C. can be reduced from 130,000 Poises to 20,000 Poises, more than 6 times, when the melt oscillates in shear at relatively low radial frequency, w=100 rad/s (16 Hz). In conclusion it is well known that the viscosity of a plastic melt can be reduced by shear thinning induced by vibration. The viscosity reduction is instantaneous and only prevails under vibration, i.e. it ceases if the vibration ceases. In other words, the viscosity reduction induced by shear thinning is not preserved and the melt is unaltered after the vibration ceases: after the melt oscillation has stopped, its Newtonian viscosity remains the same as for the initial-non vibrated melt. Therefore, the viscosity reduction induced by vibration-shear thinning is completely unstable and requires to be done while the material is injected or extruded, that is to say while the part is being shaped in a mold or a die. This implies the implementation of sophisticated vibration machinery added to traditional injection molding, blow molding or extrusion machines. Examples of such devices are described in other patents and applications (Refs 7a-7i). The same arguments can be said about the modification of the elasticity of a melt, which can be brought upon either by an increase of molecular weight or by melt vibration. The excess elasticity at a given temperature induced by the vibration condition ceases immediately at the interruption of the vibration.
In short, the use of vibration means to increase flow and modify in situ the viscosity of melts is well known. However, in order to clearly distinguish the differences between the present invention, which also uses vibration means to reduce viscosity, and the prior art, we summarize below the prior art as follows:
There are three categories of patented processes using vibration to modify the molding process and/or the properties of molded materials:
1. The common practical feature among the patents of the first category is their use of mechanical shaking/oscillation or ultrasonic vibration devices to homogenize and increase the density of the material molded, either in the liquid stage or in the solidifying stage, either at a macroscopic or microscopic level 3-6!. These references do not directly concern the use of vibration to lower the viscosity of a melt to increase their processability during conversion, nor do they address the use of packing vibration to increase the melt elasticity. PA1 2. The second category of patents and processes using vibration is based on the fact that material rheology is a function of vibration frequency and amplitude in addition to temperature and pressure. This can be put to practical use to influence diffusion and rate sensitive processes which depend on viscosity and relaxation kinetics, such as nucleation and growth of crystals, blending and orientation 1-2, 7-11!. However, as said earlier, these references do not alter the viscosity of the melts in a way which preserves the viscosity reduction, and, the alteration is entirely dependent on the activation of the vibration means which create it, while the melt is cooled and vibrated. PA1 3. In a third category, vibration is essentially used to generate heat locally by internal friction 20! or to decrease surface stresses at the wall interface between the melt and the barrel or the die to increase throughputs 12-16, 17-20!. The heat generated locally by pressure pulsation can be significant enough, in injection molding, as to avoid the premature freezing of the gate, resulting in a significant reduction of the shrinkage in the final part 20!. The significant reduction at the wall interface of the friction coefficient increases the throughput of melt flow through vibrating dies 12-16! and reduces orientational birefringence. These processes do not try to modify the viscosity of the melt per se, in a way which would be similar to a reduction of the molecular weight average of the macromolecules.
The industry would welcome a process which allows the viscosity to be significantly reduced and to stay reduced, at least for the time it is processed into a shaped article, without altering the molecular weight of the polymer and the inevitable degradation of the mechanical characteristics that results from it. Likewise, the industry would welcome a process which allows the melt elasticity to be significantly increased and stayed increased, at least for the time it is processed into a shaped article, by blow-molding or thermoforming, without the need to increase the molecular weight of the polymer and the creation of a new grade for this resin, with the inevitable costs associated with the promotion and the manufacturing of such a new grade.
In short, the industry would welcome a non-chemical process allowing the simplification of their resin line without the need to modify the processability and melt strength by varying the molecular weight average and the polydispersity of the resin into various grades.
As will become evident by reading the following disclosure, the prior art does not describe a method and apparatus capable of modifying and controlling in a significant way the viscosity and the elasticity of plastics without the need to either modify the molecular weight of the macromolecules or the addition of plasticizers, lubricants etc.